This invention relates generally to nuclear reactors and more particularly to health and safety systems and methods for removal of fission products from the coolant of such nuclear reactors. A nuclear reactor is an apparatus designed and operated for the purpose of initiating and maintaining a nuclear fission chain reaction in a fissile material for the generation of heat for power purposes. In the type of nuclear reactor described herein, fissile materials such as plutonium 239 and uranium 238, are contained within fuel elements. A plurality of fuel elements comprise a nuclear core which is structurally supported within a hermetically sealed pressure vessel. A reactor coolant, such as liquid sodium, is circulated into the pressure vessel and through the nuclear core where the heat generated by nuclear fission is transferred from the fuel elements to the reactor coolant. The heated reactor coolant exits from the pressure vessel and flows to a heat exchanger where the heat previously acquired is transferred to another reactor flow system coupled in sealing arrangement with the heat exchanger. The cooled liquid sodium exits from the heat exchanger and flows to a pump which again circulates the reactor coolant into the pressure vessel repeating the above described flow cycle. The system comprising the nuclear core, pressure vessel, heat exchanger, circulation pump, and the connecting piping is commonly referred to as the primary system.
The heat which is transferred from the reactor coolant on flowing through the heat exchanger is eventually transformed into stream which is converted into electrical energy by means of conventional steam generator, steam turbine, electrical generator apparatus. This system by which the heat is converted into electricity is known as the secondary system.
An intermediate system is sometimes included between the primary and secondary systems of the nuclear reactor for the purpose of reactor safety. Like the primary system, the intermediate system is completely closed and contains a coolant such as liquid sodium. The intermediate system comprises a heat exchanger, a pump for coolant circulation and connecting piping.
The fuel assemblies of the nuclear core of the type of nuclear reactor considered herein may include fuel elements or pins which are either vented or unvented. In the former, fuel pellets containing various isotopes of uranium and plutonium are housed within unsealed or vented cladding tubes formed from a corrosion resistant material, such as stainless steel. In the latter type of fuel element the fuel pellets are encapsulated in sealed cladding tubes. The type of fuel element that a particular nuclear reactor will use depends in part upon treatment to be afforded to a gas which is generated by the fission process. The fission gas generated by the fission process contains radioactive fission products such as cesium 137, iodine 131 and tritium. Tritium readily diffuses through the fuel element cladding at the reactor operating temperature; therefore, as regards the subsequent treatment of tritium, it is immaterial whether vented or unvented fuel elements are used. In either case tritium is released directly to the reactor coolant. If vented fuel elements are used the radioactive cesium and iodine isotopes are also released directly to the reactor coolant. The unvented fuel elements prevent direct release of the radioactive isotopes. However, in the unlikely but possible event of a rupture of a cladding tube, the unvented elements also may release cesium 137 and iodine 131. It is therefore possible for the reactor coolant to become contaminated with radioactive tritium, cesium 137 and iodine 131 not withstanding which type of fuel element is used in the nuclear core.
The health and safety problem is caused mainly by deposition of the radioactive cesium 137 and iodine 131 onto all surfaces in the primary system with which the contaminated reactor coolant comes in contact and subsequent exposure of personnel to the contaminated surfaces. This includes the surfaces of such apparatus as the reactor pressure vessel, the pressure vessel closure head, main reactor coolant circulating pumps, heat exchangers, connecting piping, valves, and like apparatus. During normal reactor operation this problem does not exist because operational personnel do not normally expose themselves to the radioactive primary system components. But, during such operations as reactor refueling reactor maintenance and primary system repairs, personnel might expose themselves to the radioactive components and a health and safety problem can exist.
The radioactive isotope tritium is a separate problem. As previously explained, at reactor operating temperatures the tritium readily diffuses through thin members such as fuel element cladding. Tritium can therefore diffuse through the tubing of the primary system heat exchanger and contaminate the intermediate system as well as the primary system which increases the possibility of personnel exposure.
In the prior art, the reactor coolant comprising liquid sodium, is partially purified of the fission product contamination by a cold trapping technique. Cold trapping is a process for removing solutes or dissolved substances from a solution. By sufficiently lowering the temperature of the solution, the solubility of the dissolved substance decreases to the level that the solute will crystallize and precipitate out of solution. The solvent or the substance within which the solute is dissolved will remain in liquid form. While this cold trapping process has been effectively used to remove non-radioactive contaminants, such as hydrogen, from a reactor coolant, it has been only marginally effective in removing radioactive fission products, such as cesium 137 iodine 131 and tritium, from a reactor coolant. Therefore, in the prior art, the health and safety problem mentioned above was not effectively elmininated.